In the fabrication or assembly of articles or parts, the parts are commonly transferred along a production or assembly line by an endless transfer conveyor to a series of work stations where each part is worked on, for example, by welding the part and/or attaching another component to the part. For example, in the construction or assembly of a motor vehicle body, a succession of the car bodies are transferred in a step-by-step or continuous manner to a work station where it is necessary to elevate each body from the conveyor so that certain operations may be performed on the body at the elevated position. It is therefore necessary to have apparatus at the work station which can receive the horizontal conveyor and also raise the body to an elevated position for a, predetermined time, after which the body is lowered back onto the conveyor.
One apparatus which has been constructed and used for elevating a part, such as a vehicle body, from a conveyor at a work station is referred to as the DCT Acculift System which is manufactured by DCT Welding and Assembly in Detroit, Mich. This system incorporates two parallel spaced elongated rails or platens which are positioned adjacent opposite edge portions of the conveyor. Each elongated rail or platen is supported for vertical movement by a pair of horizontal tracks which receive linear bearing blocks pivotally connected to a pair of crank arms mounted on opposite end portions of a horizontal shaft extending parallel to the platen. The horizontal tracks are stabilized and maintained in horizontal positions during vertical movement by vertical linear bearings while the crank arms move the tracks and platens up and down between upper and lower predetermined positions. Each crankshaft for each platen is connected to a gear box, and the two gear boxes are connected by a cross drive shaft so that the crankshafts rotate in unison. One of the gear boxes has an input shaft coupled to the output shaft of a third gear box which is driven by an electric motor. Each of the parallel spaced crankshafts is provided with an overtravel arm which limits rotation of the crankshaft in one direction, and a mechanical counter-balancing system is used in association with the crank arm on each end of each crankshaft for counter-balancing the weight of the crank arm and the corresponding bearing block and track support arms.
The present invention is directed to an improved lifter apparatus of simplified and dependable construction and which requires only one gear reducer and eliminates the need for counter-balancing systems. The apparatus of the invention also provides for synchronized and harmonic movement of the lifting rails or platens and eliminates the need for horizontal linear bearings on the mechanism which supports and moves the rails or platens between their upper and lower positions. The lifter apparatus of the invention also has an overtravel arm mounted directly on the shaft of the drive for the apparatus so that components of the lifting mechanism are not over stressed in an overtravel condition.
In accordance with one embodiment of the invention, a lifter apparatus includes two parallel spaced elongated lift rails or platens each of which is guided for vertical movement by a pair of guide assemblies including linear bearing blocks mounted on corresponding vertical guide rails attached to vertical housings. A bell crank is pivotally supported adjacent each vertical guide assembly, and the two bell cranks for each lift platen are pivotally connected to the corresponding bearing blocks by first link members and to each other by a second link member. A third link member pivotally connects each set of bell cranks for each lift platen to a corresponding crank arm mounted on a crankshaft extending horizontally and laterally below the lift platens. A drive unit is connected to one end portion of the crankshaft and includes electric drive motor mounted on a gear reducer and also coupled to an electric brake unit. The output shaft of the gear reducer is coupled to the end portion of the crankshaft and also supports an overtravel arm which is adapted to actuate an overtravel switch. The output shaft of the gear reducer is also coupled to a programmable limit switch and feedback device having a cam switch and resolver or encoder to provide precision control of the electric drive motor and brake unit.
Other features and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.